1. Field of the Invention
Embodiments of the present invention relate to gasifiers and combusters in general and to apparatus and methods related to the controlled fuel feed and combustion of various solid fuels, including biomass fuels, in particular.
2. Description of the Related Art
In smaller-scale heating systems characterized by relative simplicity there are two primary methods employed to deliver solid fuel to the burner. These are well known in the art and are referred to as bottom feed (or fed) and top feed (or fed). Bottom fed heating systems in the prior art convey solid fuel into the burner by pushing it in from the side or bottom of the burner. Top fed units are well known and drop solid fuel into a burner from above.
As is known in the prior art, heat-producing systems as described herein may include various process zones in various combinations. These zones may typically be described as drying, heating, pyrolysis, combustion and reduction and exhaust.
Fuel is added to gasifiers either in a batch mode or they may embody an automated or semi-automated feed system to deliver the fuel to the burner or combuster as described herein above. As the fuel is added to the burner it is consumed by flame and the heat is typically captured by well-known heat transfer mechanisms. The by-products of the combustion of the solid fuel such as the volatile and non-volatile gases are exhausted to the atmosphere after as much as possible of the sensible heat is removed. It will be appreciated by one skilled in the art that many of the prior art systems burn the solid fuel directly and lack a controlled or selective conversion of the solid fuel to a combustible gas. As a result of prior art systems failure to provide conversion they either fail to completely burn the volatile gases produced by the flame or use excess air to completely burn the gases and dilute the energy density of the exhaust gases available for heat extraction. Both shortcomings result in lower efficiency. Also as a result of prior art systems burning the fuel directly there is a lack of control of temperature that often results in the formation of clinker from overheating the minerals (e.g. phosphorus, sodium, etc) in the fuel.
Bottom fed burners of the prior art advantageously create less entrained ash in the exhaust and less disruption to the combustion zone thereby because the fuel bed is less disturbed as the solid fuel is pushed into the burner from beneath the combustion zone. That is to say that the fuel is less disturbed by the flame by comparison to burners where fuel is supplied to the combustion zone from the top or side of the flame area wherein ash particles become entrained in the airflow. In prior art systems where the fuel is supplied by dropping it into the combustion zone from above the fuel be must heated and volatized fairly quickly as it lands in the combustion zone. This cools the combustion zone unnecessarily and causes less efficient operation.
It will be appreciated by those skilled in the art that solid fuel heating systems of the prior art suffer from such inter-related problems as fuel delivery, ash build-up and removal and clinker formation and removal. Many bottom fed systems of the prior art include the advantage of being configurable to allow ash and clinker to be pushed out of the burner on a regular basis and are therefore inherently self-cleaning. This advantage is difficult to combine with a practical means to supply the fuel to the combustion area. The feed mechanism to supply solid fuel from a bulk storage area to the combustion area must be isolated to prevent conflagration to proceed to the supply, so called back-burn. By contrast, top fed systems of the prior art typically trap clinker and ash in the burner, which must include some means of removal or they will build up and choke out the fire. Fuel supply to top fed systems of the prior art may easily accommodate automated feed systems, such as augers, because the fuel is supplied from above the combustion area the supply is inherently isolated from back-burn.
Conventional solid fuels of the prior art include wood pellets, coal, corn, wood chips and other pelletized biomass, and typically take the form of relatively small semi-uniform shapes or particles. One problem in the prior art is that fuel delivery systems, either gravity fed or certain driven systems, is that inter-particle forces between the individual pieces of fuel cause the particles to lock-up and interrupt the delivery if fuel to the combustion area. In order to avoid such problems it is advantageous to use an automated or continuous fuel delivery system or conveying means which may typically comprise an auger. However, as used in the prior art an auger has the disadvantage that the fuel conversion cannot occur along the flights of the auger since this would be both destructive to the auger and destructive to the combustion or gasification process. The auger must release control of the solid fuel before the solid fuel reaches the combustion or gasification region and therefore the auger does a poor job of controlling the delivery of the fuel relative to the oxygen sources, ash removal point and other points relevant to the combustion or gasification process. An exemplary bottom fed burner that forcibly pushes fuel into a combustion zone is disclosed U.S. Pat. No. 5,070,798 and is incorporated herein by reference in its entirety.
It will be appreciated by one skilled in the art that the lack of controlled delivery can lead to less than complete energy conversion of the fuel and add to the problems of solid and gaseous emissions. The lack of controlled delivery leads to systems of the prior art using excessive amounts of air (oxygen) to insure complete combustion. Excessive amounts of air dilutes the production gas and can lead to a loss of sensible heat in the exhaust since the sensible heat in the excess air can not be completely removed.
A recognized shortcoming of systems in the prior art is the inherent difficulty in providing for drying and preheating of the solid fuel prior to converting the fuel into producer gas through pyrolysis and reduction or combustion of the fuel to produce heat. It will be appreciated that heating and (thereby) drying a solid fuel upstream in the flow of fuel provides better control of the combustion process. This is, in part, because the volatiles in the fuel are released in a more controlled and complete way since moisture in the fuel is deleterious to the volatilization and combustion process. In the case of the bottom feed system supplied by way of an auger there is no easy path for channeling heat from the combustion area into the solid fuel in the auger. Furthermore as discussed herein above this would increase the risk of back-burn or cause clogging in the auger flights. In the case of top fed designs back-burn prevention would be circumvented since the solid fuel would have to be heated on the opposite side of the known means of back burn prevention.
What is needed in the art is a means of conveying solid fuel through the combustion or gasification process while controlling the position of each of the process zones drying, heating, pyrolysis, combustion and reduction, and not deleteriously disturbing these zones.